This invention relates to an electrical motor and, more particularly, to a reluctance stepper motor which is operated as a synchronous motor in the microstep mode.
In some industrial applications, it is desirable to have a very high torque motor for directly driving loads, such as in industrial robots, for example. A high torque motor placed in each joint of a robot, for instance, eliminates gears or other mechanisms that have backlash. Backlash in servo systems causes dead zones, servo oscillations, and increases the wear, noise and cost and reduces the reliability of such servo systems.
Prior such reluctance stepper motors with or without a permanent magnet have not provided sufficient torque for their size and weight compared to the present invention. This has been due to a number of problems, including the fact that in reluctance type stepper motors, the magnetic flux path passes circumferentially through the rotor, thereby requiring a thicker, more massive rotor.
Furthermore, the magnetic flux path in some prior art stepper motors passes through unintended stator poles causing a reduction in the torque as well as instability in the driving amplifiers for the motor. This occurs because if an amplifier is driving one phase that is turned off, but is coupled magnetically, like a transformer, to an energized phase, an oscillation will occur between the two amplifiers, that is, between the amplifier which is actually driving one phase and the other amplifier which has been turned off. Basically, the off or reduced current driving amplifier is being misinformed about winding current and tries to correct the situation by changing the amplifier's own current away from a zero value.
Still another problem with some prior art motors of this type is that they are polarity sensitive, that is they require bipolar amplifiers to drive them in one direction or another.
Still a further problem, particularly in servo systems, is that some prior art reluctance type stepper motors have a detent torque when the power is turned off to the motor. This residual torque prevents freewheeling of the motor which is a problem in some applications such as balanced robot arms that are manually guided by the operator.